As is well known, digital telecommunication systems in North America communicate with each other using a standard basic format referred to as DS1 (e.g. T1 carrier system) or a multiple thereof wherein 24 voice channels are multiplexed into a 125 microsecond time period called a frame provided by a basic 8 kHz sampling rate. Each frame format provides for 24 channels each comprising one eight-bit word along with one frame bit. In telecommunications, signaling is a process of setting up a connection and supervising its completion as well as monitoring the sanity of the operating systems. In a DS1 data stream, signaling information is imbedded in the digital stream of bits representing the voice channels by using the least significant bit from each channel (8-bit word) in every 6th frame thus providing 24 bits every 6th frame which may be used as a signaling channel. This encoding scheme results in the use of only 7 bit words to encode the voice in those channels; however, the overall distortion is not significant. Signaling frames are provided for receiver synchronization and to indicate even and odd 6th frames thus providing a means for distinguishing between two types of signaling bits, "A" bits and "B" bits. A signaling frame in the "AB" signaling scheme is comprised of 12 frames. Frame 6 corresponds to the "A" signaling channel and frame 12 corresponds to the "B" signaling channel. Instances occur when more than two types of signaling bits are provided, as in the known extended "ABCD" signaling frame. In this signaling scheme, the "ABCD" signaling frame is a superframe comprised of 24 frames. Frames 6 and 12 correspond to the "A" and "B" signaling channels and frames 18 and 24 correspond to "C" and "D" signaling channels respectively. Generally the "A", "B", "C", and "D" signaling bits each carry a different type of signaling information.
An alternative transmission scheme (PCM-30) uses a 32 channel format with channels 0 and 16 being used as signaling channels. This transmission format uses a 16-frame signaling frame with channel 16 of each frame being partitioned in two parts each carrying one set of a, b, c, d signaling bits, and channel 0 being used to transmit other overhead information.
In contemporary telecommunication systems, it is sometimes desirable to provide an interface circuit to the carrier system for removing the signaling bits and channelizing them into a serial bit stream for further transmission along a single signal path.
As is generally known, there are numerous systems for the redundant encoding of binary data; however, none of them are suitable to provide self-synchronization to serially transmitted "ABCD" signaling bits.
This invention is directed to a method and apparatus for redundantly coding the channelized signaling bits before their channelized transmission as well as a method for decoding the received serial data stream in such a way that the "ABCD" signaling bits are properly regenerated and their association with the original channelized data is maintained.
Accordingly, it is an object of the invention to provide a system for the communication of channelized coded data which exhibits robust self-synchronizing characteristics.
It is a further object of the invention to provide a system which will recover synchronization when incorrectly coded data bits are transmitted.